The production of transportation fuels from biomass (e.g., ethanol, butanol, methane, biodiesel) continues to attract interest, due to the low cost and wide availability of biomass, and because biofuels may be used to displace the use of fossil fuels. For example, bioethanol may be blended into gasoline at predetermined concentrations (e.g., 10%).
First generation biofuels, also referred to as conventional biofuels, are made from biomass that contains sugar, starch, or vegetable oil (e.g., food crops). For example, ethanol may be produced by fermenting sugars that are easily extracted from sugar crops (e.g., sugar cane or sugar beets), or may be produced by fermenting sugars derived from starch-based feedstocks (e.g., corn grain, barley, wheat, potatoes, cassava). However, the diversion of farmland or crops for first generation biofuel production has led to much debate about increased food prices and/or decreased food supplies associated therewith. In addition, there are concerns related to the energy and environmental impact of these production processes.
Second generation biofuels, also referred to as advanced biofuels, wherein the biomass contains lignocellulosic material and/or is obtained from agricultural residues or waste (e.g., corn cobs, corn stover (e.g., stocks and leaves), bagasse, wood chips, wood waste), may allay some of these concerns. For example, when bioethanol produced using second generation processes (i.e., also referred to as cellulosic ethanol) is derived from agricultural waste or residue, its production should not affect the food supply. Accordingly, tremendous effort is currently being expended to advance cellulosic ethanol production processes.
Lignocellulosic biomass typically contains cellulose, hemicellulose and lignin, each of which is commonly present in plant cell walls. Cellulose (e.g., a type of glucan) is an unbranched chain polysaccharide including hexose (C6) sugar monomers (e.g., glucose). Hemicellulose is a branched chain polysaccharide that may include different pentose (C5) sugar monomers (e.g., xylose and arabinose) in addition to glucose. Lignin is a complex organic polymer, which typically includes cross-linked phenol polymers. Although generally insoluble in water at mild conditions, lignin may be soluble in varying degrees in dilute acid or base alkali. The ratio and/or structure of these components (e.g., cellulose, hemicellulose, and lignin) may vary depending on the biomass source.
Unfortunately, since cellulose, hemicellulose, and/or lignin found in lignocellulosic biomass typically is structured to resist degradation, it can be difficult to release sugars from this naturally recalcitrant material. Accordingly, a pretreatment step often is used to open up the structure of the lignocellulosic material and/or to make it accessible for enzymes. The pretreated material, which is more amenable to enzymatic hydrolysis, may then be converted to sugars, such as glucose, with enzymes, and then fermented with a microorganism. The fermentation product (e.g., an alcohol such as ethanol or butanol), may then be recovered (e.g., distillation) and used as a transportation fuel or other product.
Some examples of pretreatments used to prepare the lignocellulosic biomass for enzymatic hydrolysis include dilute acid pretreatment, alkali pretreatment (e.g., lime), ammonia fiber expansion, autohydrolysis (e.g., hot water extraction that does not require the addition of acid or base), steam explosion, organic solvent, and/or wet oxidation.
One type of pretreatment that has been studied is sulfur dioxide (SO2)-catalyzed steam pretreatment. Sulfur dioxide is a gas, which when dissolved in water, may be referred to as sulfurous acid. Sulfur dioxide and/or sulfurous acid may be a suitable catalyst for acid-catalyzed steam pretreatment since it may produce a more digestible substrate. In addition, it may produce less and/or fewer inhibitors and/or inactivators relative to dilute sulfuric acid pretreatment.
However, pretreatment with sulfur dioxide has been limited. First, sulfur dioxide costs more than sulfuric acid (e.g., on a weight basis). Second, sulfur dioxide is a weaker acid than sulfuric acid, so more acid required. As a result of the increased cost and amount of acid required, and/or as a result of environmental concerns, it may be desirable to recover and/or the reuse the sulfur dioxide, which adds to the cost. To date, the performance with sulfur dioxide has not offset these additional costs.
In particular, the use of sulfur dioxide has not been adopted in commercial ethanol plants, where the use of sulfuric acid has been preferred. In particular, the observed hydrolysis performance of sulfur dioxide catalyzed steam pretreated lignocellulosic material (e.g., which has been similar to the hydrolysis performance of sulfuric acid catalyzed steam pretreated lignocellulosic material) has not been exceptional enough to outweigh the added cost (e.g., the cost of sulfur dioxide is greater than the cost of sulfuric acid) and/or increased process complexity (e.g., careful handling to address safety and environmental concerns) of using sulfur dioxide.